Under the February 13 agreement reached at talks with the US, South Korea, Japan, Russia and China, North Korea agreed to end its nuclear programme in exchange for heavy fuel oil or equivalent aid.
The deadline for North Korea to "disable" its key nuclear complex at Yongbyon and disclose all its nuclear activities is the end of December.
|"The president ... stressed the need for North Korea to come forward with a full and complete declaration of their nuclear programmes"|
Gordon Johndroe, White House spokesman
Johndroe said: "The president reiterated our commitment to the six-party talks and stressed the need for North Korea to come forward with a full and complete declaration of their nuclear programmes, as called for in the September 2005 six-party agreement."
Earlier, Hill, who is in Beijing to brief Chinese officials on his three-day trip to North Korea, had made no mention of the letter.
Hill said the North was moving to cripple the reactor and other units at Yongbyon so they would be difficult to restart.
But disagreements remain over what should appear in the comprehensive declaration of nuclear activities that Pyongyang has promised.
Before meeting the Chinese diplomats, Hill said that one of the points of dispute was North Korea's efforts to enrich uranium, a way of making nuclear material that does not rely on reactors.
Hill said: "We've had a lot of discussions with them about uranium enrichment."
He said that the United States had "very good evidence" that North Korea had bought enrichment technology and had received assistance from Pakistan.
The February agreement would still leave North Korea to take the crucial steps of irreversibly dismantling Yongbyon and handing over any nuclear weapons materials.
But Pyongyang may now miss the year-end deadline for the disarmament steps, South Korea's foreign minister indicated.
According to a spokesman for his ministry, Song Min-soon said: "We are aiming for the initial end-of-the-year deadline, but we may need to be a little more flexible."
Recently, the Bush administration has played down claims about how advanced North Korea was in enrichment.
However, Hill suggested that even if those efforts were fruitless or dormant, North Korea had to make a full disclosure.
He said: "We want to be completely sure they don't have any ongoing programme. Being clear about what's happened is also a means for us to build a future relationship."